Despite the widespread use of marijuana, relatively little is known about the long term effects of the drug on neurocognitive and neurophysiologic function. The proposed investigation emerges from our studies of neuropsychological effects of marijuana in college students and adults, as well as from our previous investigations utilizing neuropsychological paradigms and functional magnetic resonance imaging (fMRI) to assess changes in regional cerebral activity in response to cognitive challenges. Neuropsychological assessments indicate that selected cortical functions, especially attentional and memory processing, are diminished after heavy cannabis use. Moreover, we have recently studied chronic marijuana smokers after a 28 day abstinence period, and found that performance on measures of memory and attention remained significantly reduced. The neural mechanisms underlying these cognitive deficits, the extent to which these functions may recover, and the time course of recovery remain to be clarified. To date, no study has incorporated focal neuroimaging using cognitive challenge paradigms and a lengthy period of monitored abstinence from the drug. Fast scan, fMRI techniques have increased the spatial and temporal for functional imaging beyond what has previously been available. We now propose to examine regional cerebral activation during cortical challenge paradigms in three study groups following 1 and 28 days of a monitored abstinence from marijuana. The three study groups will be comprised of 60 current heavy long-term smokers of marijuana, 60 former heavy long-term smokers of marijuana, and 60 light smokers of marijuana. This project will provide important new information regarding the type, presence, and magnitude of the effects of cannabis on brain function. Further, the identification of neural processes which demonstrate improvements in cognitive activation following abstinence may be critical for the development of intervention strategies, the design of new treatments, and decisions regarding public policy.